the easy way to keep your cells balanced is...active balancing

[Tyler702]

The OKS BMS (and most BMS's) is only a passive balancer. That means that it only burns off the power in the highest cell. In the case of that particular BMS (and again, most) it is not capable of burning off much. Something like 200mA max. That isn't enough to get these big 200Ah+ cells balance quickly, but it might work over several charge / discharge cycles.

Gotcha! I'm only a 100Ah bank right now, but interesting how the xioxiang app depicts (flashes) the highest cell(s) and the 1 lowest cell when balancing. The low guy will come up a bit while the high one(s) come down a bit. That led me to believe it was bleeding one to the other. In my case cell 7 has been my slacker. So actually it may be burning off the other 7 cells at intervals, till all 7 are down to within my threshold difference of the slacker?

 

[Daddy Tanuki]

so far no answers on what value of a resistor one should use to pre-charge these units?

 

[Gérard DEXPERT]

 

[toms]

Sky when you used a resistor to pre-charge what was the value of the resistor? Also any body got an idea on what value resistor one should use on a 48 volt bank to precharge the inverter to prevent arcing and current inrush damage?

The REC precharge module i use for a 48V system uses a 66ohm resistor.

The resistor is to protect the contactor, not the capacitors.

 

[Steve_S]

I use Chargery BMS8T which has passive balancing, balance ON during charge & storage. start @ 3.30 and allow for 20mv diff.
I also have a QNBBM-8S Active Balancer on every pack which do the heavy lifting. By end of my float cycle, cells in all packs tend to be between 6mv to 20mv pending on pack.

<- QNBBM-8S
BMS harness wired to Balancer and Balancer out to cells with 14g copper & tinned rings.
* Suggest using wire Ferules, easier, or twist the wires together very well before inserting.






Alternatives for connecting an Active Balancer:

The HELLA block with ATS fuses is one option that is likely best used with active balancers above 5A capacity. Will impact IR readings.
The MOLEX without a fuse is another good alternative, a Ferule is a good idea as always with stranded wire but not "necessary". Although solder tipping stranded wire is a good idea. NB, these have small through screw holes in the body so they can be mounted easily.
BOTH are available with various pin outs.
REMEMBER, BMS has an extra wire for Neg on cell harness.


Hope it helps, Good Luck.

 

[Diysolar123]

Gotcha! I'm only a 100Ah bank right now, but interesting how the xioxiang app depicts (flashes) the highest cell(s) and the 1 lowest cell when balancing. The low guy will come up a bit while the high one(s) come down a bit. That led me to believe it was bleeding one to the other. In my case cell 7 has been my slacker. So actually it may be burning off the other 7 cells at intervals, till all 7 are down to within my threshold difference of the slacker?

when I was testing my bms for balancing I watches the app balance "flash" for days...it did nothing and I tested two units and they acted the same.
fyi, I had set balance to always be on and disconnected the charger so its a best case scenario.
If the balance current (which for the bms is just a passive resistor drain) had been over 40ma of current that is well within the range of accurate readings from the current meter I have, it was not remotely that high (the limits on my meter is in the +/- 10-20ma range; the display goes down to a ma but that means nothing hehe).

as a followup my cells after all this time are <drum roll as I log into nodered to check> ... lowest cell 3.400, highest cell 3.402

 

[Diysolar123]

so far no answers on what value of a resistor one should use to pre-charge these units?

I did a long winded post on this topic some time back...

Inverter Capacitor Pre-charging

I've watched Will Prowse and other's on Youtube pre-charging the capacitors on their inverters before connecting them to the battery. Generally, they use a high power resistor to ease the current in without a big spark. That's a great idea but it got me thinking... the right kind of incandescent...

diysolarforum.com

 

[Daddy Tanuki]

The REC precharge module i use for a 48V system uses a 66ohm resistor.

The resistor is to protect the contactor, not the capacitors.

Thanks, I have sent a inquiry to the maker to find out what they recommend. They replied with an asinine question. not surprised. they were basically what would you need a resistor for? well if I am asking the question that means that somebody else fried your garbage by not using one, and then on their own figured out something. Mind you, I was loading the big caps for stereos back in the 90's to keep them from popping. so if that was needed back then, then its probably needed now. still waiting upon a reply. we will see how responsive they are...

I have institutional dislike for vendors who don't answer or who hoard info.

R

ken

 

[Daddy Tanuki]

I did a long winded post on this topic some time back...

Inverter Capacitor Pre-charging

I've watched Will Prowse and other's on Youtube pre-charging the capacitors on their inverters before connecting them to the battery. Generally, they use a high power resistor to ease the current in without a big spark. That's a great idea but it got me thinking... the right kind of incandescent...

diysolarforum.com

thanks for the link. will read and digest.

 

[toms]

Thanks, I have sent a inquiry to the maker to find out what they recommend. They replied with an asinine question. not surprised. they were basically what would you need a resistor for? well if I am asking the question that means that somebody else fried your garbage by not using one, and then on their own figured out something. Mind you, I was loading the big caps for stereos back in the 90's to keep them from popping. so if that was needed back then, then its probably needed now. still waiting upon a reply. we will see how responsive they are...

I have institutional dislike for vendors who don't answer or who hoard info.

R

ken

I’m not surprised by the manufacturers response. I haven’t seen a capacitor that requires a limited charge current.

The only reason i use a precharge is to protect the contactor or charge source (battery) - i’ve never personally seen a capacitor damaged by high current.

I’m curious exactly what component of the active balancer is being damaged by high current?

 

[Daddy Tanuki]

I’m not surprised by the manufacturers response. I haven’t seen a capacitor that requires a limited charge current.

The only reason i use a precharge is to protect the contactor or charge source (battery) - i’ve never personally seen a capacitor damaged by high current.

I’m curious exactly what component of the active balancer is being damaged by high current?

when dealing with a 1 farad cap for an audio system they did it to keep the batteries, fuses and breakers intact. for smaller items like what I am dealing with now, I can see it for a multitude of reasons... not the least to keep you from scorching the contact area and causing resistance. think about it, if you get a big honking spark across the area where your ring terminal contacts the buss bar, or the battery terminal itself, it would create resistance. in the case of the above poster, he claimed to have burst a cap when he plugged it in. I could see that in smaller caps getting charged from a battery that can put out 300 x its C1 for a couple of seconds. the in-rush could be fatal for smaller caps. back the day Amp makers required their trained people to do the same...but that was 12 volt and not 48. I just needed a quick and easy way to calculate the size for my system, and thanks to DIYsolar123 I have an idea from which to start.

it is possibly unneeded but how much is a 5 ohm by 500 watt resistor? pennies on the dollar and safer.. hell I can buy three different sizes going down from there and still not spend a dollar... cheap insurance.

 

[Horsefly]

it is possibly unneeded but how much is a 5 ohm by 500 watt resistor?

I don't know if that is a typo, but a 500 watt resistor is way bigger than is easily available. You can get them, but they are very costly.

I think people over-do how small a resistor they use. The idea is to charge the capacitors in the front-end of the inverter, and that won't really take too long or too much current. Recall that the capacitors will appear as a direct short for a split second. For that split second, the watts through the pre-charge resistor would be (V * V)/R = (48 * 48)/R. If you use a more nominal 40 ohm that would be 57W. That wattage would be for only a few milliseconds, as the current will go down quickly as the capacitors charge. So in that case I think you could get by with a 50W resistor. Much cheaper.

 

[snoobler]

Higher ohms/lower watts is just as effective and even cheaper. A 500Ω, 10W resistor does the trick too (48V system).

 

[Daddy Tanuki]

I must have transposed a decimal place while I was figuring that up last night. now I am not even sure how I came up with that number.... thats what happens when you try to do math with the help of a friend helping you like Jim....Beam..

 

[Horsefly]

I must have transposed a decimal place while I was figuring that up last night. now I am not even sure how I came up with that number.... thats what happens when you try to do math with the help of a friend helping you like Jim....Beam..

Don't be hard on yourself - Your math was actually correct: 48 * 48 / 5 = 460 watts. So if you actually do use a 5 ohm resistor, it will be carrying 460 watts for a tiny fraction of a second. You just don't need to use a resistor that small.

So Jim Beam did you good! (But use a bigger resistor )

 

[Gazoo]

so far no answers on what value of a resistor one should use to pre-charge these units?

I use these to precharge the caps in my 24 volt 2000 watt inverter. Precharge for 10 seconds and quickly hook up the positive cable. Zero spark.

 

[snoobler]

I use these to precharge the caps in my 24 volt 2000 watt inverter. Precharge for 10 seconds and quickly hook up the positive cable. Zero spark.

Those are definitely under-rated, but as has been mentioned, the current drops off so quickly, it's probably not a risk.

 

[Gazoo]

Those are definitely under-rated, but as has been mentioned, the current drops off so quickly, it's probably not a risk.

If they were underated for this application wouldn't I still see a spark The resistors were suggested here and that's why I bought them. A 25 watt resistor might be better but I have used the same 5 watt 50 ohm resistor a few times with no problems.

 

[Horsefly]

If they were underated for this application wouldn't I still see a spark The resistors were suggested here and that's why I bought them. A 25 watt resistor might be better but I have used the same 5 watt 50 ohm resistor a few times with no problems.

No, it wouldn't cause a spark. The underrating that @snoobler is talking about is that there is more than 5W going through the resistor, if only for a fraction of a second. For your 24V system, the resistor is getting a bit under 12W. If you did that continuously, the resistor would probably pop on you. However, for the short amount of time the pre-charge takes, it won't hurt anything.

 

[chrisski]

I will get a 25 watt, 30 ohm resistor to pre-charge a capacitor for the bigger inverter I will get for my 24 volt upgrade.

25 Watt Ceramic Cement Power Resistor, 30 ohms: Amazon.com: Industrial & Scientific

25 Watt Ceramic Cement Power Resistor, 30 ohms: Amazon.com: Industrial & Scientific

www.amazon.com

I did notice Will said they were a dollar, but of course I can’t find those on Amazon for that cheap, but these are not typically available at the local walmart.

Think it wa earlie run this thread that someone said the LED light resistor, The 50 watt 6 ohm resistors, won’t work. Shame those LED resistors don’t work since Those are available at most Auto Parts stores.